This invention relates to the method of fabricating a multi-wall tubular member and particularly to such a multi-wall tubular member having a hardened inner wall and a high strength outer wall.
In various fluid systems, the piping and other tubular members may require an inner surface or wall having special specifications and requirements which are different than that of the surrounding outer wall. In such applications, a dual or multi-wall tubular member can be formed from two separate telescoped tubular members. Concrete pumping represents one application in which a dual wall specification is encountered. The abrasive characteristic of the concrete being pumped requires that the inner surface of the piping having a highly abrasive resistance characteristic. Generally, where metal piping is used, this requires an inner wall of a very hardened metal. Concrete pumping however also involves relatively high pressures, particularly pressure surges. This requires a pipe having a very high tensile strength to operate satisfactorily over long periods of time under normal pumping pressures. For example, pipe units which would advantageously be constructed as in such a composite pipe are truck mounted elongated pipe section for pumping concrete, such as shown in U.S. Pat. No. 3,860,175 which issued on Jan. 14, 1975 and is assigned to a common assignee with the present application. Various other applications encounter similar differential characteristic requirements. Oil well piping systems, nuclear power plants and various chemical flow systems require various special specifications for interior of the piping and contrast to a normally high strength requirement for the piping generally. Thus, one solution which has been disclosed and used commercially involves forming a composite multi-wall pipe from individual pipes or tubular members mounted in telescoped relation and specially processed to establish a firm interfaced engagement and connection between the two pipes. The inner pipe can then be formed with the necessary characteristics demanded by the particular product being transported. The outer pipe can be formed as a high strength member such that the pipe unit can withstand the pressures encountered under operating conditions.
U.S. Pat. No. 4,497,673 which issued Feb. 5, 1985 discloses a multi-wall pipe for applications in those systems requiring a hardened inner liner or wall. In accordance with the teaching of that patent, the tubular members are originally constructed to permit telescoping of the tubular members. After assembly, the inner liner is flame hardened causing the expansion of the inner liner into engagement with the outer liner or shell to form a composite member. The patent specifically teaches that the inner liner is provided with sequential heating and hardening to progressively expand the pipe and provide the desired interface interconnection. The flame hardening requires entry into the pipe with an appropriate torch unit for heating of the inner pipe to the necessary degree for hardening. U.S. Pat. No. 4,449,281 which issued May 22, 1984 discloses a alternate system. In this system or disclosure, cold water is introduced into the pipe, with the ends of the inner pipe sealed. The water is pressurized to expand the inner pipe. Thereafter, an induction heating unit is moved over the pipe unit and specially arranged to heat only the exterior pipe of the two telescoped pipes. The inner water chambers are further pressurized to expand and produce a bond therebetween or the firm engagement therebetween. A similar system is disclosed in U.S. Pat. No. 4,598,857 which issued on July 8, 1986. In this disclosure, after the induction heating, a mechanical working device is applied within the expanded exterior pipe and inner pipe to collapse the pipes to form a composite pipe. U.S. Pat. No. 3,579,805 which issued on May 25, 1971 discloses a system using freezing of the pipe unit to effect interengagement between the pipes of a multiple layered pipe unit. Other methods of forming composite dual walled pipes or multiple wall pipes are disclosed in the following U.S. patents:
______________________________________ U.S. Pat. No. Issue Date Inventor ______________________________________ 3,579,805 05-25-1971 Kast 4,030,711 06-21-1977 Siller 4,322,894 06-01-1982 Whistler 4,332,073 06-01-1982 Yoshida ______________________________________
Nothwithstanding, the activity and the efforts in the prior art in developing a multi-wall tubular member and pipe unit, the present systems are relatively costly and also have certain limitations which prevent long life operation.